(1) Field of the Invention
The present invention relates to automatic gain control systems which automatically control the gain of digital modulating signal, more particularly to dual AGC control system which has double gain control loop of RF AGC and IF AGC.
(2) Description of the Related Art
As digital modulation techniques used in order that signal level may generally transmit digital signal data (namely, symbol) expressed with binary methods, digital modulation method such as Phase shift Keying (PSK), Quadrature Amplitude Modulation (QAM), and Orthogonal Frequency Division Multiplexing (OFDM), are known.
In modulating equipment which adopted these digital modulation techniques, orthogonal modulation by analog carrier is performed using two symbol of I and Q which are sampled and quantized at predetermined time interval.
Further, thus the modulated signal is converted to radio frequency (RF), such as VHF band and UHF band, and is sent out to them.
In digital demodulating equipment, after tuning in signal of desire wave and carrying out frequency conversion to Intermediate Frequency (IF), carrier component is eliminated, compare is carried out to predetermined symbol level, and the nearest symbol level is judged a playback symbol value.
Since signal level of received signal is fluctuated according to field streng that the time, signal level of recovery signal is always fixed using Automatic Gain Control (AGC).
Hereafter, conventional automatic gain control system is explained with reference to FIG. 13. FIG. 13 shows a block diagram of conventional automatic gain control system.
After amplifying RF signal provided from the terminal 601 in the RF AGC amplifier 602, it is provided to the 1st frequency converter 604.
The 1st frequency converter 604 transforms output of the RF AGC amplifier 602 into predetermined 1st intermediate frequency based on frequency control signal provided to the terminal 603.
After carrying out band limitation of the output of the 1st frequency converter 604 by the 1st filter 605, it is provided to the 2nd frequency converter 606.
The 2nd frequency converter 606 transforms output of the 1st filter 605 into predetermined 2nd intermediate frequency based on frequency control signal provided to the terminal 603. After carrying out band limitation of the output of the 2nd frequency converter 606 by the 2nd filter 607, it is provided to the IF AGC amplifier 608. The IF signal 609 amplified by the IF AGC amplifier 608 is provided to the orthogonal wave detector 610 and the signal level detector 614.
The orthogonal wave detector 610 provides I and Q component signal which eliminated the carrier component to the demodulator 611. The demodulator 611 recovers the transmitted digital signal data from two component signals, I and Q, and provides it to the error corrector 612.
The error corrector 612 corrects the error in digital signal data, and provides it to the output terminal 613.
After detecting signal level of IF signal 609 provided from the IF AGC amplifier 608, the signal level detector 614 provides the error between the signal level and predetermined level, i.e., signal level error signal agcerr, to the loop filter 615. The loop filter 615 integrates signal level error signal agcerr, and provides the AGC signal 616 which eliminated the noise component to the comparator 617.
The comparator 617 compares the AGC signal 616 with predetermined level, i.e., the delay point level. When the AGC signal 616 is smaller than delay point level, the control signal is provided to the IF AGC amplifier 608. When the AGC signal 616 is larger than delay point level, the control signal is provided to the RF AGC amplifier 602.
The operation of the conventional automatic gain control system shown in FIG. 13 is explained.
The RF AGC amplifier 602 and the IF AGC amplifier 608 have the gain property that the gain becomes small, when the control signal provided from the comparator 617 becomes large, respectively. Moreover, the signal level detector 614 has the characteristics that the output becomes large, when the signal level of IF signal 609 becomes larger than predetermined signal level.
For this reason, since the output of the signal level detector 614 becomes small when the signal level of IF signal 609 is small, it is controlled so that the gain of the IFAGC amplifier 608 or the RF AGC amplifier 602 becomes large and the signal level of IF signal 609 becomes large.
Moreover, since the output of the signal level detector 614 becomes large when the level of the input signal is large, it is controlled so that the gain of the IF AGC amplifier 608 or the RF AGC amplifier 602 becomes small and the signal level of IF signal 609 becomes small.
Thus, the signal level of IF signal 609 is controlled to desired level by changing the gain of the IF AGC amplifier 608 or the RF AGC amplifier 602. This control is called automatic gain control.
It is decided on delay point level using which of the IF AGC amplifier 608 and the RF AGC amplifier 602 the automatic gain control is carried out.
When the AGC signal 616 is smaller than delay point level, the gain of the RF AGC amplifier 602 is fixed and the automatic gain control is performed by changing the gain of the IF AGC amplifier 608.
When the automatic gain control signal is larger than delay point level, the gain of the IF AGC amplifier 608 is fixed and the automatic gain control is performed by changing the gain of the RF AGC amplifier 602.
Thus, changing IF AGC and RF AGC according to the signal level of the input signal is performed in order to take the large control range of AGC, maintaining low noise characteristics. Namely, on the premise that the total gain of the IF AGC amplifier 608 and the RF AGC amplifier 602 is fixed, in the case of low gain of the RF AGC amplifier 602 and high gain of the IF AGC amplifier 608, the noise factor (NF) becomes higher than the reverse case.
For this reason, when signal level is low, it is more advantageous to fix RF AGC on the maximum gain in respect of noise characteristics.
On the other hand, if the automatic gain control is applied only by the IF AGC amplifier 608, since the control range of AGC is limited by the dynamic range of the IF AGC amplifier 608, the range becomes narrow. Then, when signal level is high, the automatic gain control range can be made wide by applying AGC using the RF AGC amplifier 602.
As explained above, the conventional automatic gain control system has the characteristics of having the wide control range by the low noise.
However, the conventional automatic gain control apparatus causes the following faults, when the high disturbance wave of signal level is in the adjacent channel.
Since there is no filter which limits the band of the signal before RF AGC, with the RF AGC amplifier 602, the signal of the adjacent channel is also amplified with the signal of the desire wave. On the one hand, with the latter filter, band limitation of the automatic gain control signal is carried out, and it is generated according to the level of the signal of the desire wave.
For this reason, if the disturbance wave of high signal level is in the adjacent channel, the gain of the RF AGC amplifier 602 is too high, and may become the saturation state. The saturation state of the RF AGC amplifier 602 causes the distortion of the signal, and produces the remarkable degradation for the performance of the demodulator. Since the RF AGC amplifier 602 is saturated when the worst, the demodulation of the signal may not be made.
Thus, in the conventional automatic gain control system, when the adjacent disturbance wave of high signal level existed, there was a problem that the performance of the demodulator degraded remarkably.
The inventions provide in part an automatic gain control method and its system which are the few circuit scale, and have low noise characteristics and high adjacent disturbance oppression characteristics, and do not have the need for the tuning.
A claimed method comprises: amplifying a radio frequency signal received; converting the radio frequency signal amplified into an intermediate signal; amplifying the intermediate signal; outputting an error signal which is deference between a level of the intermediate signal amplified and a predetermined level; controlling a gain of amplification of the intermediate signal according to the error signal outputted; controlling a gain of amplification of the radio frequency signal according to the error signal outputted; and, changing a delay point level, the delay point level being a boundary value of whether the gain of the intermediate frequency signal amplification is controlled, or the gain of the radio frequency signal amplification is controlled.
According to the present invention, even if it can change the delay point level which performs the switching of the IF amplification gain control and the RF amplification gain control and only the desired signal is receivable with sufficient selectivity when there is an unnecessary strong electric wave near received wave frequency.
Moreover, in other invention, the change of the delay point level is automatically carried out so that the error rate may become small by the error rate detection step.
According to this invention, even if there is an unnecessary electric wave strong against the frequency near the received frequency, only the desired signal is receivable with sufficient selectivity, quickly and automatically.